Plunger solenoid

ABSTRACT

A solenoid having a longitudinally movable plunger and a coil operable on energization to move the plunger toward a fixed pole piece. A housing holds the fixed pole piece and surrounds the coil. A sleeve of ferromagnetic material is mounted on the housing end plate and surrounds a portion of the plunger to provide a magnetic path for the useless leakage and fringing fluxes of the solenoid, thereby reducing saturation within the plunger. The enlarged cross sectional area in the pole piece accommodates the total of the useless and useful flux without excessive saturation. The leakage flux flows in a radial path from the pole piece, the sleeve and/or plunger, as the case may be, across the coil to the outside cylindrical wall thereby shunting the working gap. The fringing flux shunts the useful flux gap from the plunger to the fixed pole piece. The fixed pole piece can have a removable head facing the end of the plunger. An opposing external force, such as a spring, engageable with the plunger and housing operates to bias the plunger away from the head.

United States Patent 1 Field PLUNGER SOLENOID William J. Field, Minneapolis, Minn.

[731 Assignee: Laketown Manufacturing Corporation, Inc., Waconia. Minn.

[22] Filed: Apr. 1, 1974 [2]] Appl. No.: 456,558

[75] Inventor:

Primary ExaminerHarold Broome Attorney, Agent, or FirmL. Paul Burd [57] ABSTRACT A solenoid having a longitudinally movable plunger June 17, 1975 and a coil operable on energization to move the plunger toward a fixed pole piece. A housing holds the fixed pole piece and surrounds the coil. A sleeve of ferromagnetic material is mounted on the housing end plate and surrounds a portion of the plunger to provide a magnetic path for the useless leakage and fringing fluxes of the solenoid, thereby reducing saturation within the plunger. The enlarged cross sectional area in the pole piece accommodates the total of the useless and useful flux without excessive saturation. The leakage flux flows in a radial path from the pole piece, the sleeve and/or plunger, as the case may be, across the coil to the outside cylindrical wall thereby shunting the working gap. The fringing flux shunts the useful flux gap from the plunger to the fixed pole piece. The fixed pole piece can have a removable head facing the end of the plunger. An opposing external force. such as a spring, engageable with the plunger and housing operates to bias the plunger away from the head.

25 Claims, 8 Drawing Figures OQ Cu PHENTED JUN 17 I975 SHEET PATENTEDJUN17 I975 ("I 3, (190.587

.FIG'-5 F 5 5 PLUNGER SOLENOID BACKGROUND OF INVENTION Conventional pulltype solenoids are designed so that the energy generated within the normal open gap, or working gap, between the plunger and the fixed pole piece is sufficient to overcome the forces that are keeping them apart. These forces may be the return spring force, the weight of the plunger and the parts attached to the plunger such as a valve, switch or the like. A substantial amount of flux over and above the amount required for the working gap must be supplied because of flux leakage paths. These paths consist primarily in the radial space from the plunger and from the fixed pole piece to the outer tubing shell through the winding and the fringing flux path from the plunger to the fixed pole piece outside the confines of the working gap. These flux leakages contribute to the magnetic saturation of the parts and add nothing to the required force and energy necessary to operate the solenoid. In some solenoids, the leakage could be from IV: to 4 times the flux in the working gap. Saturation means that the flux density in the ferromagnetic material reaches a state where excessive magnetomotive force is required for an incremental increase in magnetic flux. The result is that the pull-in conditions for operation when the plunger is in the extended position cannot be reached.

The normal magnetization characteristics for various ferromagnetic materials vary considerably. Materials such as low carbon steel are reasonably priced and have good ratio of flux density divided by magnetic intensity in the area where the force and motion are expected to start. For a flux density range from 10 to I kilomaxwells per square inch, the range of the magnetomotive force is from 2.5 to 20 ampere turns per inch of iron path which is below the magnetic saturation for this material. From l to I35 kilomaxwells per square inch of iron area, the range of the magnetomotive force is from I00 to I000 ampere turns per inch of iron. This is the range in which the magnetic saturation occurs. Magnetic saturation occurs when the increment in the flux density is insufficient to attain the force necessary to overcome the force that is keeping the plunger from being pulled in. The number of ampere turns used in the solenoid increases the heating and requires a larger unit. In many installations, there are limitations of size and shape of the solenoid or the diameter of the plunger, which prevents a larger or more massive solenoid. The present invention overcomes these design limitations of the prior art.

SUMMARY OF THE INVENTION The invention is directed to a pull-type solenoid having means to carry leakage fluxes so that a smaller size solenoid will have the power of a conventional size so lenoid. The solenoid of the invention has coil means operable to produce a magnetic field. A fixed pole piece is surrounded by the coil means and is mounted on a housing end plate. A housing surrounds the coil and has a second end plate accommodating a movable plunger means. The plunger means is movable relative to the fixed pole piece in response to electrical energization of the coil means. A ferromagnetic flux conductor means is located between the coil means and the plunger means to provide a path for leakage and fringing fluxes and to minimize useless flux passage through the plunger means. The fixed pole piece has a transverse cross sectional diameter larger than the transverse cross sectional diameter of the plunger means and provides a ferromagnetic path for leakage fluxes, reducing saturation and minimizing the passage of use less flux through the plunger means. The fixed pole piece has a head facing the plunger means. The head may be a removable member and constructed of non corrosive magnetic material. The head in one form of the invention has a cone-shaped recess adapted to accommodate a cone-shaped projection on the end of the plunger means.

An object of the invention is to provide a pull-type solenoid with means to carry leakage fluxes so that a smaller size solenoid requiring a relatively long stroke will have the pull force of a larger conventional sole noid. A further object of the invention is to provide a solenoid with a fixed pole piece having a cross sectional area larger than the cross sectional area of the plunger to increase the ferromagnetic path so that useless fluxes will not aggravate saturation in the useful flux path. A still further object of the invention is to provide the fixed pole piece of a solenoid with removable heads that accommodate the ends of the movable plungers.

IN THE DRAWINGS FIG. 1 is a plan view of the plunger pull-in solenoid of the invention mounted on a support;

FIG. 2 is an elevational view of the right end of FIG.

FIG. 3 is an enlarged longitudinal sectional view taken along the line 33 of the solenoid of FIG. 1;

FIG. 4 is a cross sectional view taken along the line 44 of FIG. 3;

FIG. 5 is a longitudinal sectional view similar to FIG. 3 of a modified solenoid according to the invention;

FIG. 6 is a sectional view taken along the line 66 of FIG. 5;

FIG. 7 is a longitudinal sectional view similar to FIG. 3 of a second modification of the solenoid; and

FIG. 8 is a sectional view taken along the line 8-8 of FIG. 7.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the drawing, there is shown in FIGS. 1 and 2 an elongated cylindrical solenoid indicated generally at 10 mounted on a support 11. Support 11 has a pair of oppositely located holes 12 and I3 suitable to accommodate bolts or other fasteners used to attach the support on a fixed member. The midportion of support 11 has a large central threaded hole 14 for accommodating the solenoid.

As shown in FIGS. 3 and 4, solenoid 10 has a cylindrical outer cover or shell 16 having an opening for electrical leads 17. An annular boot or protector 18 attached to the cover 16 surrounds the leads 17. The shell 16 is mounted on a pair of end plates 19 and 21. End plate 21 has an outwardly directed annular nipple 22. The outer surface of the nipple has threads 23 adapted to be threaded into hole 14 of the support 11. End plate 21 has a cylindrical center hole accommodating an elongated cylindrical sleeve 24. Sleeve 24 has a first end 24A located in the hole of end plate 2] and a second end 248 located approximately in the midsection of the solenoid. Sleeve 24 does not extend over the active air gap 37 when the plunger 27 is in the extended position as shown in FIG. 3 so that the sleeve 24 does not provide a shunting path for the useable flux. Sleeve 24 is made of ferromagnetic material to provide a separate path for useless leakage and fringing fluxes away from the plunger area. An elongated brass tube or cylinder 26 is located within sleeve 24. The cylinder 26 provides a sleeve bearing for an elongated cylindrical plunger 27. The plunger 27 is slidably mounted in cylinder 26. The end of the plunger 27 projected from end plate 21 carries a snap ring 28. Ring 28 has an inner annular portion located in a groove 29 in the plunger. A spring 31 concentrically positioned about the plunger engages the snap ring 28 and outer end of end plate 21 to bias the plunger to its out position.

A fixed pole piece 32 is attached to the end plate 19. Pole piece 32 extends inwardly in an axial direction toward plunger 27. Pole piece 32 has an inwardly directed cylindrical portion or head 32A which fits into the end of the cylinder 26. Pole piece 32 has a larger diameter dimension 33 which is sufficient to accommodate the useful flux plus the useless fluxes without excessive saturation. The diameter dimension 33 is not necessarily equal to the outside diameter of the sleeve 24. Fixed pole piece 32 has a middle section with the diameter 33. The transverse cross sectional area of this seciton of pole piece 32 is larger than the transverse cross sectional area of plunger 27. The forward section 32A of pole piece 32 has a flat inside face 34. The inner end of plunger 27 has a flat face 36 parallel to and facing the flat pole face 34. Faces 34 and 36 are spaced from each other and form the working flux gap 37 of the solenoid. The solenoid coil or winding 38 is located within the outer shell 16 and extends between end plates 19 and 21. The leads 17 are connected to the coil 38. The cross sectional area of sleeve 24 provides a magnetic path for the leakage flux across the radial gap between sleeve 24 and shell 16 in the length c of sleeve 24 from end plate 21.

The cross sectional area of the fixed pole piece 32 at dimension 33 is proportioned to provide a magnetic path for all the radial leakage flux across the gap between the outside cylindrical wall of pole piece 32 and shell 16 coming to the cylindrical length a plus the fringing flux across the working gap 37 and the useful or active flux passing through the plunger length and through the working gap 37.

The sum of the lengths a, b and c is equal to the longitudinal distance between end plates 19 and 21. The lengths a, b and c are proportioned to minimize the leakage fluxes and the fringing flux and to optimize the useful flux through the working gap 37. As shown in FIG. 3, lengths a and b are about equal to each other and length c is about one and one-half the length of 0. Other proportions can be used to optimize the useful flux.

The fixed gap length for flux across the space from the plunger end plate 2] and sleeve 24 across brass member 26 to the plunger 27 for the length c plus length d of end plate 21 is proportioned to optimize the useful flux in working gap 37 consistent with mechanical strength required for the structure and the freedom of movement for the plunger. The ampere turn drop across this space depends inversely on the ratio ofc and d divided by the radial distance between plunger 27 and sleeve 24. The greater the ratio the smaller the drop.

Referring to FIGS. and 6, there is shown a modified solenoid indicated generally at 39. The parts of solenoid 39 that correspond to solenoid are identified with the ..ne reference numerals having the suffix A. The solenoid 39 has a fixed pole piece 32A attached to the end of plate 19A. An axial hole 41 extends into the fixed pole piece 32A. A head 42 is releasably attached to the inner part of the fixed pole piece 32A. Head 42 can be a non-corrosive metal member, as stainless steel. Head 42 has an axial projection or threaded member 43 which is threaded into hole 4]. The head 42 has an enlarged annular body which fits into the inner end of tubular member 26A. Head 42 has a flat face 44 facing the flat end 36A of plunger 27A. The space 37A between faces 36A and 44 is the flux gap and working distance of the solenoid. The flat face plunger 27 is used for relatively large force, short stroke applications. In these applications, the ratio of the square root of the force in pounds divided by the plunger traverse is in the range from 60 to 350. Threaded member 43 is illustrative only and should not be construed as limiting the method of fastening. Also, it is not necessary to have the piece releasable.

Referring to FIGS. 7 and 8, there is shown a second modification of the solenoid of the invention indicated generally at 46. Solenoid 46 is similar to solenoid 10. Parts of solenoid 46 that are identical with the parts of solenoid 10 have the same reference numerals with the suffix B. Solenoid 46 has an end plate 198 carrying the fixed pole piece 328. Fixed pole piece 32B has an axial hole 47 extended into the pole piece on the end thereof. A head 48 of non-corrosive ferromagnetic metal or material, as stainless steel, is located adjacent the inner end of the pole piece 323. Head 48 has a threaded projection 49 that is threaded into the hole 47 to thereby releasably connect the head 48 to the pole piece 328. Head 48 has a cylindrical outside diameter of a size which fits into the inner end of tube 26B. The head 48 illustrated here has a cone-shaped recess or face 51 open toward the plunger 27B. The inner end of plunger 278 has a cone-shaped end 52 complementary to the shape of the recess 51. The flux gap between the recess 5] and cone-shaped face of plunger 27B is indicated by arrow 53. The conical plunger 27B is used where a small force, long stroke ratio is desired. The angle of the cone is related to the force-stroke ratio. A cone angle 0 of 45 results in ratio ranges from 15 to 60. A cone angle 6 of 60 results in ratio ranges from 5 to 15. The threaded member 49 is illustrative only and should not be construed as limiting the method of fastening. Also, it is not necessary to have the piece releasable.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A solenoid comprising: coil means for producing a magnetic field, fixed pole piece means surrounded by the coil means, said fixed pole piece means having a cylindrical portion with a generally uniform diameter surrounded by the coil means, housing means accommodating the coil means and holding the fixed pole piece means at one end section of the coil means, plunger means movable from an extended position to a contracted position relative to the fixed pole piece means in response to energization of the coil means, said plunger means having a cross sectional area smaller than the cross sectional area of the cylindrical portion of the fixed pole piece means, and means of ferromagnetic material located between the coil means and plunger means to provide a path for leakage fluxes and minimize useless flux passage through the plunger means, said means of ferromagnetic material including a ferromagnetic sleeve surrounding a section of the plunger means when the plunger means is in the extended position, said sleeve being mounted on the housing means and extended toward the fixed pole piece means, said sleeve having an end longitudinally spaced from the fixed pole piece means a longitudinal distance greater than the longitudinal distance between the end of the plunger means and the fixed pole piece means, said coil means surrounding said sleeve.

2. The solenoid of claim 1 including: non-ferromagnetic tubular means mounted on the fixed pole piece means and extended along the longitudinal axis of the coil means, said plunger means being slidably located in said tubular means, said ferromagnetic sleeve being located about the tubular means.

3. The solenoid of claim 1 wherein: said fixed pole piece means has a longitudinal length about one-half the longitudinal length of the sleeve surrounded by the coil means.

4. The solenoid of claim 3 wherein: the sleeve is longitudinally spaced from the end of the cylindrical portion of the fixed pole piece means a distance about the length of said cylindrical portion of the fixed pole piece means.

5. The solenoid of claim 1 wherein: said housing means has an end plate, said sleeve being mounted on said end plate.

6. The solenoid of claim 1 wherein: the fixed pole piece means has a removable head facing the plunger means.

7. The solenoid of claim 6 including: non-ferromagnetic tubular means mounted on the housing means and extended along the longitudinal axis of the coil means, said tubular means having an end located about the head.

8. The solenoid of claim 6 wherein: the head has a flat face and the plunger means has a flat end facing the flat face of the head.

9. The solenoid of claim 6 wherein: the head has a cone-shaped recess and the plunger means has a coneshaped projection adapted to fit into the cone-shaped recess of the head.

10. The solenoid of claim 1 wherein: the fixed pole piece means has a head facing the plunger means, said head being of non-corrosive ferromagnetic material.

11. The solenoid of claim 1 wherein: said fixed pole piece means, means of ferromagnetic material and longitudinal spaced between the fixed pole piece means and means of ferromagnetic material having lengths proportioned relative to each other to minimize the leakage and fringing fluxes and optimize the useful flux through the working gap of the solenoid.

12. A solenoid comprising: coil means for producing a magnetic field, fixed pole piece means surrounded by the coil means, housing means accommodating the coil means and holding the fixed pole piece at one end section of the coil means. plunger means movable relative to the fixed pole piece in response to energization of the coil means, said fixed pole piece means having a cylindrical section with a generally uniform transverse cross sectional area larger than the transverse cross sectional area of the plunger means to provide a path for leakage fluxes and to minimize useless flux passage through the plunger means, said coil means surrounding the cylindrical section of the fixed pole piece means, and sleeve means of ferromagnetic material surrounding a portion of the plunger means and secured to the housing to provide a path for leakage fluxes and minimize useless flux passage through the plunger means, said sleeve means having an end spaced from the fixed pole piece means a distance greater than the distance between the end of the plunger means and the fixed pole piece means when the plunger means is in the extended position.

13. The solenoid of claim 12 wherein: the fixed pole piece means has a head smaller in cross sectional area than the section of the fixed pole piece means, said head having a surface facing an end of the plunger means.

14. The solenoid of claim 13 wherein: said head has a cone-shaped recess and said plunger means has a cone-shaped end adapted to fit into the cone-shaped recess of the head.

15. The solenoid of claim 13 wherein: said head includes means to releasable connect the head to the section of the fixed pole piece.

16. The solenoid of claim 13 including: non-ferromagnetic tubular means mounted on the housing and extended along the longitudinal axis of the coil means, said tubular means having an end portion mounted on the fixed pole piece means, said head being located within said end portion of the tubular means.

17. The solenoid of claim 13 wherein: said head is a non-corrosive ferromagnetic material.

18. A solenoid comprising: coil means for producing a magnetic field, fixed pole piece means surrounded by the coil means, housing means accommodating the coil means and holding the fixed pole piece means at one end section of the coil means, plunger means movable from an extended position to a contracted position relative to said fixed pole piece means in response to energization of the coil means, and means for ferromagnetic material located between the coil means and the plunger means to provide a path for leakage fluxes and minimize useless flux passage through the plunger means, said means of ferromagnetic material including a ferromagnetic sleeve surrounding a section of the plunger means when the plunger means is in the extended position, said sleeve being mounted on the housing and extended toward the fixed pole piece means, said sleeve having an end longitudinally spaced from the fixed pole piece means a longitudinal distance greater than the longitudinal distance between the end of the plunger means and the fixed pole piece means, said coil means surrounding said sleeve.

19. The solenoid of claim 18 including: non-ferromagnetic tubular means mounted on the fixed pole piece means and extended along the longitudinal axis of the coil means, said plunger means being slidably located in said tubular means, said sleeve being located about the tubular means.

20. The solenoid of claim 19 wherein: the fixed pole piece has a head facing the plunger means, said tubular means having an end mounted on said head.

21. The solenoid of claim 19 wherein: the fixed pole piece has a removable head facing the plunger means, said tubular means having an end mounted on the removable head.

22. The solenoid of claim 18 wherein: the fixed pole piece has a head, said head having a flat face and the plunger means has a flat end facing the flat face of the head.

gap of the solenoid.

25. The solenoid of claim 1 wherein: the sleeve surrounding the section of the plunger means does not extend to the vicinity of the working gap of the solenoid so that the sleeve shunts only the leakage and fringing fluxes from passing through the plunger means thereby increasing the useful flux and magnetic force in the working gap at the beginning of the stroke of the plunger means. 

1. A solenoid comprising: coil means for producing a magnetic field, fixed pole piece means surrounded by the coil means, said fixed pole piece means having a cylindrical portion with a generally uniform diameter surrounded by the coil means, housing means accommodating the coil means and holding the fixed pole piece means at one end section of the coil means, plunger means movable from an extended position to a contracted position relative to the fixed pole piece means in response to energization of the coil means, said plunger means having a cross sectional area smaller than the cross sectional area of the cylindrical portion of the fixed pole piece means, and means of ferromagnetic material located between the coil means and plunger means to provide a path for leakage fluxes and minimize useless flux passage through the plunger means, said means of ferromagnetic material including a ferromagnetic sleeve surrounding a section of the plunger means when the plunger means is in the extended position, said sleeve being mounted on the housing means and extended toward the fixed pole piece means, said sleeve having an end longitudinally spaced from the fixed pole piece means a longitudinal distance greater than the longitudinal distance between the end of the plunger means and the fixed pole piece means, said coil means surrounding said sleeve.
 2. The solenoid of claim 1 including: non-ferromaGnetic tubular means mounted on the fixed pole piece means and extended along the longitudinal axis of the coil means, said plunger means being slidably located in said tubular means, said ferromagnetic sleeve being located about the tubular means.
 3. The solenoid of claim 1 wherein: said fixed pole piece means has a longitudinal length about one-half the longitudinal length of the sleeve surrounded by the coil means.
 4. The solenoid of claim 3 wherein: the sleeve is longitudinally spaced from the end of the cylindrical portion of the fixed pole piece means a distance about the length of said cylindrical portion of the fixed pole piece means.
 5. The solenoid of claim 1 wherein: said housing means has an end plate, said sleeve being mounted on said end plate.
 6. The solenoid of claim 1 wherein: the fixed pole piece means has a removable head facing the plunger means.
 7. The solenoid of claim 6 including: non-ferromagnetic tubular means mounted on the housing means and extended along the longitudinal axis of the coil means, said tubular means having an end located about the head.
 8. The solenoid of claim 6 wherein: the head has a flat face and the plunger means has a flat end facing the flat face of the head.
 9. The solenoid of claim 6 wherein: the head has a cone-shaped recess and the plunger means has a cone-shaped projection adapted to fit into the cone-shaped recess of the head.
 10. The solenoid of claim 1 wherein: the fixed pole piece means has a head facing the plunger means, said head being of non-corrosive ferromagnetic material.
 11. The solenoid of claim 1 wherein: said fixed pole piece means, means of ferromagnetic material and longitudinal spaced between the fixed pole piece means and means of ferromagnetic material having lengths proportioned relative to each other to minimize the leakage and fringing fluxes and optimize the useful flux through the working gap of the solenoid.
 12. A solenoid comprising: coil means for producing a magnetic field, fixed pole piece means surrounded by the coil means, housing means accommodating the coil means and holding the fixed pole piece at one end section of the coil means, plunger means movable relative to the fixed pole piece in response to energization of the coil means, said fixed pole piece means having a cylindrical section with a generally uniform transverse cross sectional area larger than the transverse cross sectional area of the plunger means to provide a path for leakage fluxes and to minimize useless flux passage through the plunger means, said coil means surrounding the cylindrical section of the fixed pole piece means, and sleeve means of ferromagnetic material surrounding a portion of the plunger means and secured to the housing to provide a path for leakage fluxes and minimize useless flux passage through the plunger means, said sleeve means having an end spaced from the fixed pole piece means a distance greater than the distance between the end of the plunger means and the fixed pole piece means when the plunger means is in the extended position.
 13. The solenoid of claim 12 wherein: the fixed pole piece means has a head smaller in cross sectional area than the section of the fixed pole piece means, said head having a surface facing an end of the plunger means.
 14. The solenoid of claim 13 wherein: said head has a cone-shaped recess and said plunger means has a cone-shaped end adapted to fit into the cone-shaped recess of the head.
 15. The solenoid of claim 13 wherein: said head includes means to releasable connect the head to the section of the fixed pole piece.
 16. The solenoid of claim 13 including: non-ferromagnetic tubular means mounted on the housing and extended along the longitudinal axis of the coil means, said tubular means having an end portion mounted on the fixed pole piece means, said head being located within said end portion of the tubular means.
 17. The solenoid of claim 13 wherein: said head is a non-corrosive ferromagnetic material.
 18. A solenoid comprising: coil means for producing a magnetic field, fixed pole piece means surrounded by the coil means, housing means accommodating the coil means and holding the fixed pole piece means at one end section of the coil means, plunger means movable from an extended position to a contracted position relative to said fixed pole piece means in response to energization of the coil means, and means for ferromagnetic material located between the coil means and the plunger means to provide a path for leakage fluxes and minimize useless flux passage through the plunger means, said means of ferromagnetic material including a ferromagnetic sleeve surrounding a section of the plunger means when the plunger means is in the extended position, said sleeve being mounted on the housing and extended toward the fixed pole piece means, said sleeve having an end longitudinally spaced from the fixed pole piece means a longitudinal distance greater than the longitudinal distance between the end of the plunger means and the fixed pole piece means, said coil means surrounding said sleeve.
 19. The solenoid of claim 18 including: non-ferromagnetic tubular means mounted on the fixed pole piece means and extended along the longitudinal axis of the coil means, said plunger means being slidably located in said tubular means, said sleeve being located about the tubular means.
 20. The solenoid of claim 19 wherein: the fixed pole piece has a head facing the plunger means, said tubular means having an end mounted on said head.
 21. The solenoid of claim 19 wherein: the fixed pole piece has a removable head facing the plunger means, said tubular means having an end mounted on the removable head.
 22. The solenoid of claim 18 wherein: the fixed pole piece has a head, said head having a flat face and the plunger means has a flat end facing the flat face of the head.
 23. The solenoid of claim 18 wherein: the fixed pole piece has a head, said head having a cone-shaped recess and the plunger means has a cone-shaped projection adapted to fit into the cone-shaped recess of the head.
 24. The solenoid of claim 18 wherein: the length of the fixed pole piece means, the length of the ferromagnetic sleeve and longitudinal space between the fixed pole piece means and the sleeve are proportioned relative to each other to minimize the leakage and fringing fluxes and optimize the useful flux through the working gap of the solenoid.
 25. The solenoid of claim 1 wherein: the sleeve surrounding the section of the plunger means does not extend to the vicinity of the working gap of the solenoid so that the sleeve shunts only the leakage and fringing fluxes from passing through the plunger means thereby increasing the useful flux and magnetic force in the working gap at the beginning of the stroke of the plunger means. 